In wireless, or cellular, communication networks, it is important that the downlink and uplink frame timing be synchronized between a base station and mobile devices served by the base station. The mobile devices connected to the base station use the same transmit and receive frequencies.
To ensure that there is no interference between the mobile devices, the mobile devices are assigned time slots or sub-channel frequencies depending on the type of multiplexing (e.g., Time Division Duplex (TDD) or Frequency Division Duplex (FDD)). In either case, frame timing must be precisely maintained over radio links between the base station and the mobile devices.
As shown in FIG. 1, timing must be aligned between the radio equipment controller (REC)10 and the radio equipment (RE) 12 such that the first sample of downlink (DL) radio frame is transmitted into the air, i.e. reach the Antenna Reference Point (ARP) 14, at the same time as the REC's transmit reference point (BFN@TRP)16. The allowed timing error in the radio is typically 20 nsec. This means that the first sample may reach the ARP 14 at BFN@TRP 16 with ±20 ns delay.
On the uplink (UL) the first sample of the UL radio frame is the one received at the ARP 14 at BFN@TRP. The allowed timing error in the radio is also 20 nsec. This means that the sample marked by the radio as the first in the UL radio frame must have entered the ARP 14 at BFN@TRP±20 ns.
For downlink path delay compensation, the REC advances the downlink baseband data such that it arrives at the radio's antenna reference (ARP)14 point precisely when it starts out at the REC's transmit reference point 16 (BFN@TRP). The REC computes the compensation using the measured downlink delay to the radio and the radio downlink processing delay it receives from the radio during CPRI path setup.
For the uplink path delay compensation, the radio uses path delay information, it receives from the REC, and it's internal uplink processing delay to advance the CPRI data such that the arrival time of this uplink data is aligned with the outbound data. It is up to the radio to provide further internal timing compensation for each carrier and account for variations due to frequency, operating temperature and component age on both the uplink and downlink data paths.
During radio production both the downlink and uplink data paths must be precisely calibrated for timing alignment. The in-equipment delay or Toffset obtained at production and stored at each radio is then used for the synchronization process. For this to work, a large amount of delay calibration data must be stored in non-volatile memory. A radio must be re-calibrated after factory repairs and this process is complex and time consuming. In addition, with change in frequency, temperature and component aging, the stored in-equipment delay can change which results in timing errors. Although the equipment is designed to allow certain timing errors, wide variations can still occur. If such large variations occur, further calibration is required in the field, which is expensive, time consuming, and introduces maintenance problems.
As such, there is a need for an automatic delay calibration technique which eliminates the need to store calibration data with each radio.